In the field of high-quality motion picture projection systems such as those available from Imax Corporation, great emphasis is placed on image brightness. One factor in achieving high levels of image brightness is the reflective characteristics of the screen on which images are projected. Special so-called "high gain" reflective paints have been developed that are applied to the image receiving surface of the screen. In the case of 3-D projection systems in which the projected light is polarized, the paint also has the effect of preserving linear polarization of the light.
At least in the case of screens other than domed screens, the paint typically is applied to the screen using a spray gun as the screen is erected in a theatre. A single coat of paint is applied to the front surface of the screen by moving the gun across the screen as it is withdrawn upwardly from a housing on the floor of the theatre.
The present inventors have recognized that it is important to maintain a constant paint spray fan width as the paint contacts the screen, over the entire screen surface. If the width of the spray changes due to bulges in the screen (in or out), the same thickness of paint may not be applied to all areas of the screen. This affects the appearance of the screen and can lead to visible vertical stripes. In addition, a narrower paint spray would have a greater concentration of paint than a wider area, and vice versa; this will affect the drying time of the paint which can exacerbate the appearance of stripes.
So far as is known, the prior art fails to provide a solution to this problem. It has been proposed to use mechanical sensors to space a spray gun from a surface to receive paint. For example, U.S. Pat. No. 4,296,317 (Kraus) shows a spray gun which is fitted with a wheel adjacent the gun nozzle for maintaining a constant distance between the paint gun nozzle and the surface of a work piece to be painted. U.S. Pat. No. 4,108,105 (Wiggins) discloses a paint spray system in which a mechanical sensor is used to determine the width of a workpiece and then control the distance of the paint nozzles from the workpiece. However, mechanical sensors that contact the surface to be painted are not acceptable in the case of motion picture screens because of the risk of marking or otherwise damaging the screen.
Examples of other prior art patents of which applicant is aware are U.S. Pat. Nos. 4,278,046 (Clarke et al.) and 5,203,923 (Hartman).
An object of the present invention is to provide a painting method and apparatus which is designed to permit the application of substantially uniform paint coatings to motion picture projection screens and other vertical surfaces.